Switch-on-to-Fault Schemes in the Context of Line Relay Loadability


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The Protection System Review Program – Beyond Zone 3 document, issued in August 2005.

Appendix B stated: 

When determining the switch on to fault element settings, each utility shall verify that the schemes will not operate for emergency loading conditions.

Unfortunately, this requirement was not accompanied by adequate guidance. This reference document is intended to provide transmission protection system owners with guidance for the review of existing switch-on-to-fault schemes to ensure that those schemes do not operate for non-switch-on-to-fault conditions or under heavily stressed system conditions. This document also provides recommended practices for application of new switch-on-to-fault schemes. 

Related Article: Power System Selectivity: The Basics Of Protective Coordination

Switch-on-to-fault (SOTF) schemes1 are protection functions intended to trip a transmission line breaker when closed on to a faulted line. Dedicated SOTF schemes are available in various designs, but since the fault-detecting elements tend to be more sensitive than conventional, impedance-based line protection functions, they are generally designed to be “armed” only for a brief period following breaker closure. Depending on the details of scheme design and element settings, there may be implications for line relay loadability. This paper addresses those implications in the context of scheme design.

SOTF Schemes Application

SOTF schemes are applied for one or more of three reasons:

  1. When an impedance-based protection scheme uses line-side voltage transformers, SOTF logic is required to detect a close-in, three-phase fault to protect against a line breaker being closed into such a fault. 
  2. When an impedance protection scheme uses line-side voltage transformers, SOTF current fault detectors may operate significantly faster than impedance units when a breaker is closed into a fault anywhere on the line. The dynamic characteristics of typical impedance units are such that their speed of operation is impaired if polarizing voltages are not available prior to the fault.
  3. Regardless of voltage transformer location, SOTF schemes may allow high-speed clearing of faults along the entire line without having to rely or wait on a communications-aided tripping scheme. Current or impedance-based fault detectors must be set to reach the remote line terminal to achieve that objective.

SOTF Line Loadability Considerations

The SOTF protection must not operate assuming that the line terminals are closed at the outset and carrying up to 1.5 times the emergency line rating, subject to applicable technical exceptions. 

There is also a concern, based on actual events which have occurred in connection with blackouts, for the undesired operation of SOTF schemes when a breaker is closed into a line which is already energized from another terminal. The SOTF protection must not operate when a breaker is closed into an unfaulted line that is energized with a voltage exceeding 0.85 PER UNIT from the remote terminal

SOTF and Automatic Reclosing

Direct-tripping high-set instantaneous phase overcurrent - Dead-line reclosing voltage supervision is often set as low as 0.2 or 0.3 per unit. With some lines, the setting must be significantly higher due to the capacitively-induced voltage from other circuits occupying the same right-of-way. With appropriate consideration of dead-line reclosing voltage supervision, there are no coordination issues between SOTF and automatic reclosing into a de-energized line.

Related Article: Fundamentals of Generator Protection

Closing into an Energized Line - Reclosing logic for the “follower” terminal may include (in addition to a sync-check element) both live-bus and live-line voltage detectors set at or above the lowest system voltage for which automatic reclosing is deemed desirable. A setting in the vicinity of 0.8 PER UNIT is not unusual.

Simultaneous High-Speed Reclosing of two or More Terminals of a Line- high-speed reclosing may elude the SOTF safeguards which exist for time-delayed dead-line reclosing because substantial power flow can be expected within a very short period of time after closing into the deadline. For some SOTF schemes, the success of high-speed reclosing may depend solely upon the current fault detector setting and the through-load flowing at the instant that the breakers are closed. If the setting is insufficient to prevent tripping of one or both terminals, one or more time-delayed reclosures may yet be successful.

Typical Reclosing Schemes 

SOTF schemes are an important element of transmission system protection. However, unless they are carefully applied, they may compromise the ability of the transmission system to tolerate heavy loading. This paper has presented the relationship between the intended purpose of SOFT schemes and introduced methods by which SOFT schemes can be made more tolerant of heavy load conditions.


  • Switch-on-to-Fault Schemes in the Context of Line Relay Loadability | Download
  • A Technical Document Prepared by the System Protection and Control Task Force Of the North American Electric Reliability Council 

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